Lamp ballast

ABSTRACT

A solid-state ballast for an electric discharge lamp, the ballast including an unfiltered full-wave rectifier, a transistor inverter for increasing the frequency and a transformer for coupling the lamp.

United States Patent Inventor Frederick W. Paget Hamilton, Mass. Appl. No. 788,524 Filed Jan. 2, 1969 Patented May 18, 1971 Assignee Sylvania Electric Products Inc.

LAMP BALLAST 5 Claims, 3 Drawing Figs. v U.S. Cl......v v 315/99, 315/258, 331/113(.1), 321/65 Int. Cl .1". HOSb 39/00,

1-105b 41/02, H05b 41/24 Field of Search 315/100 [56] References Cited UNITED STATES PATENTS 2,923,856 2/1960 Greene et a1. 315/138 2,964,676 12/1960 Davies et al. 315/9 8 3,005,130 10/1961 Schwartz 315/206 3,146,406 8/1964 Wilting 331/113 3,354,350 11/1967 Kappenhaggen et a1 315/205 Primary Examiner-Herman Karl Saalbach Assistant ExaminerWm. H. Punter AttorneysNorman J. OMalley and Laurence Burns ABSTRACT: A solid-state ballast for an electric discharge lamp, the ballast including an unfiltered full-wave rectifier, a

transistor inverter for increasing the frequency and a trans former for coupling the lamp.

Patented May 18, 1971 3,579,026

FIG.!

FREDERICK W. PAGET INVENTOR ATTORNEY LAMP BALLAST FIELD OF INVENTION This invention relates to apparatus for starting and operating electric discharge lamps.

SUMMARY OF PRIOR ART Apparatus for this purpose is commonly called a ballast, and has heretofor been of the electromagnetic type.

For a lamp operated at the usual commercial frequency of 60 cycles per second, an electromagnetic ballast is heavy and cumbersome. Various efforts have been made to eliminate the electromagnetic ballast, or to reduce its size, by the use of semiconductors in various connections.

BRIEF SUMMARY OF INVENTION I have discovered, however, that the combination of a fullwave rectifier, an inverter, and a very small transformer is quite effective as a ballast for discharge lamps, and in-the case of a pair of fluorescent lamps, can weigh as little as one pound in contrast to the customary electromagnetic ballast for the same lamps, which weighs 8 pounds. The ballast of mu invention is also much smaller than the regular electromagnetic ballast. I

The inverter used can be a push-pull transistor oscillator, fed by a substantially unfiltered rectified direct current source, and is connected to the lamp through a very small high frequency transformer having some leakage reactance and containing a feedback winding to be connected directly or'indirectly to the control electrodes of the transistors. The same small transformer had windingsfor supplying filament voltage for the cathodes of the lamps for rapid start operation.

The use of unfiltered rectified direct current appears to be necessary to make my device fully effective. The-frequency of the oscillation appears to depend on the applied voltage. Accordingly, it rises as the rectified wave rises and eventually shifts over suddenly to a much higher frequency as the oscillator changes its mode of oscillation. The mean frequency for the oscillations during the first part of the wave is about kilocycles per second, with about 40 kilocycles per second for the peak portion of the wave.

A cup-type transformer is very effective as the transformer between the inverter and the lamps, and can be made very small, say about 1%diameter and about three-fourths inch long, with a weight of only a few ounces. The leakage reactance portion of the secondary winding can be placed at the end of the core inside the cup and a magnetic annular ring placed between the winding and the other windings to supply a magnetic leakage path.

I have discovered that the wave form of the lamp current and the heating of the various componentsof the currentcan be considerably reduced by connecting a small inductance coil in series with the line before the rectifier,-that is, between one of the line terminals and an input terminal to the rectifier. The reason for this is not understood at present, but its effectiveness is quite clear.

BRIEF DESCRIPTION OF DRAWINGS Other objects, advantages, and features of the invention will be apparent from the following specification; taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagram of a circuit embodying the invention;

FIG. 2 is a perspective view of a transformer used which can be used in the circuit; and

FIG. 3 is a longitudinal section through the center of said transformer.

DESCRIPTION OF SPECIFIC EMBODIMENT In FIG. 1, the line 1, 2 is connected to a bridge rectifier circuit 3 made up of crystaldiodes 4, 5, 6, 7. A small iron-core inductance 8 is in series with the line terminal 1 and the bridge rectifier 3.

The rectifier output across lines 9, 10 feeds an inverter referred to as 11, including the condenser 12, which is of a value insufficient to filter the direct current output from the rectifier 3.

The inverter circuit 11 is of the push-pull type using the transistors 13, 14; the input and output of the transistors being coupled electromagnetically through the transformer 15. This transformer comprises an iron core 16 and the coils 17, 18, 19, 20. Each of the coils 17 and 18 is connected across the transistor from the collectors 21, 22 to the emitters 23, 24, with the condenser 12, across which is the bridge rectifier 3, connected in series with the common line connecting the emitters 24, 23 to the common connection between coils l7 and 18. Coils 19 and 20 are connected through the common connection 25 to the emitters 23, 24 of the transistors 13, 14 in series with the diode 26 shunted by the condenser 27 to provide the proper biasing voltage during oscillation. A connection directly from the bridge line 9 through a resistance 28 is also made to the diode 26 to provide a bias voltage at starting. The noncommon end of coil .20 is connected to the base of transistor 13 through'resistance 32, and anoncommon end of coil 19 is connected to the base 30 emitter of transistor 14 through the resistance 31. Inductance 33 is connected between the two base electrodes 29 and 30.

A condenser 47 is connected across the outside ends of coils l7 and 18, that is, between the collector 21 and collector 22. Diode 34 is connected across .coil 17 and the condenser 12 in series, and the diode '35 is similarly connected to coil 18, the two diodes being connected to pass current in opposite directions. The lamps 37, 38 are connected across the outside ends of series connected coils l7 and 18 with the coil 39 connected to increase the starting voltage and to act as an inductance after the lamps start. The two lamps 37 and 38 are connected in series, and the electrodes 40, 41, 42, 43 are heated by the coils 44, 45 and 46 of the transformer.

The inductance 33 is in effect an open circuit during the part of the cycle in which the transistors 13, 14 conduct current, and short circuits the coils 19, 20 through resistors 31, 32 during the remainder of the cycle. The diodes 34, 35 short out the emitter-collector circuits during the part of the cycle after the other transistor turns off and provide a path for the current due to voltage in the transformer coil, either 17 or 18, which is not in circuitwith the transistor during that part of the cycle.

As shown in FIGS. 2 and 3, transformer core 16 is made of two cup-shaped parts, 50, 51 with their open ends butted against each other, and held together by the nylon screw 52, which passes through a central cylinder 53, 54 in each cup and coaxial with it, the center cylinders also being butted together. The screw 52 can be of plastic, with a suitable nut 55 on the end protruding from the cup 51. The various coils are wound on a plastic bobbin which fits over the central cylinder 53, 54. Coils l7, l8, 19, 20, 44, 45, 46 are wound on the bobbin over the central cylinder 53 and the other coil 39 on the bobbin over central cylinder 54 in the other cup. An annular ring 56 of magnetic material fits over the end of central cylinder 54 in cup 51 and acts as a magnetic shunt to provide a leakage path for flux passing between coil 39 and the other coils, In this way, the coil 39 has enough leakage reactance to act as a choke coil. The air gap can be fixed in part by the clearance between the edges of the annular ring and the inside of the cup 51.

The annular'ring can be powdered iron in a phenolic binder, the binder being about 5 percent of the total. The use of such a magnetic material makes the air gap less critical, since part of the gap is in the annular ring itself, for the phenolicmaterial coats and insulates the particles of powdered iron.

The cups 50, 51 are made of a power frequency ferrite which had a permeability of about 60.

Coil 18 is nearest the closed end of cup 50, with coil 17 next to it, and then coils 44, 45, 46 one over the other. Annular plastic spacers fit between coils 17 and 18, and between 18 and the three coils mentioned above. The lead-in cables 60, 61 contain all the lead-in wires to coils17, 18, 44,..45, 46 and lead-in cable 62 contains the lead-in wires to coil 39. The leadin wires extend through holes in the ferrite cups 50, 51. The ends of the cup can have several holes drilled in them for impregnating. For example, six holes about Va inch diameter have been used.

The transformer described is about 1% inch outside diameter with an /a inch wall thickness. The cups are each about inch length measuring from the outside surface. The end wall is also about A inch. The outside diameter of the central cylinder is about A inch, with about ya inch central hole for the screw 52. The annular ring 56 is about Vs inch thick and about 1% inch outside diameter, with a central hole about 1/32 more than one-half inch.

This transformer is especially effective in the circuit explained above.

Although a specific embodiment has been described, various modifications and variations will be apparent to a worker skilled in the art, from the above specification, without departing from the spirit and scope of the invention as set forth in the claims.

In FIG. 1, the lamps 37, 38 are of the usual 40 watt rapid start fluorescent about 48 inches long and 1% inches in diameter. The filamentary cathodes 40, 41, 42, 43 are connected to windings 44 45, 46, of about three turns each on core 16,

which will be described both in connection with FIGS. 2 and 3. Coil 39 has about 100 turns, and coils l9 and 20, four turns each, with about 100 turns also on coils 17 and 18.

Capacitor 47 can be of 0.02 mfd. capacitor 12 of about 0. l mid, and capacitor 27 of about 0.2 mfd.

The transistors 13 and 14 are of the 2 N 3902, NPN silicon type. Diode 26 is an IN 4383, as are also diodes 2, 5, 6, 7. Diodes 34 and 35 are type IN 4385, capable of withstanding about 600 volts back voltage.

Resistances 31, 32 are of 18 ohms and resistance 28 of 8000 ohms. The inductance 8 is an iron core choke coil of about 1.5 millihenries. It acts as a radio frequency filter between the rectifier and the line to prevent radio interference. In addition, however, it unexpectedly increases the efficiency of the ballast as a whole. Inductance 33 is of the square loop type, of 16 turns on a core with a flux capacity of900 maxwells.

A condenser 47 is connected across the outside ends of the transformer coils l7, 18 when their inner ends are connected together in series. The words inside" and outside" merely refer to the position of the ends in the circuit, and not to their geometrical position in the actual transformer.

in operation, the rectifier 3 feeds raw rectified direct current to the inverter 11, which changes it to alternating current of a frequency much higher than the line, say 40,000 cycles per second. This is then fed to the lamp through a small transformer 15 including inductance 39. The two lamps are in series. A single lamp can be used if desired. No condenser is used across either lamp.

The back-to-back rectifying diodes 34, 35 together with condensers 47 and 12 help to reduce any voltage buildup from the transistors and transformer circuit while operating. The transformer 15 and condenser 47 provide a tank circuit for the oscillations of the inverter. The voltage of the power line connected to l, 2 is volts Hertz.

Although a particular embodiment of the invention has been described above, various alternations and modifications will be apparent to a worker skilled in the art, without departing from the spirit and scope of the invention as set forth in the claims.

Iclaim:

1. An electric discharge lamp circuit operating from an alternating current line, said circuit comprising: a full-wave rectifier for supplying a substantially unfiltered rectified direct current means to an inverter; an inverter to which said wave is applied, the output frequency of the inverter being substantially higher than that of the alternating current line said inverter including two transistors connected in push-pull; a transformer for coupling at least one fluorescent lamp to the output of said inverter and in ut and output circuits of said inverter being coupled throug a coil together to produce oscillation at a frequency substantially higher than that of the alternating current line, and an inductance to short circuit said coil when the transistors are not carrying current.

2. The circuit of claim 6, in which the alternating current line has a frequency of about 60 cycles per second, and the output frequency of the inverter is above 8,000 kilocycles per second.

3. The circuit of claim 6 in which the transformer is of the pot type and has an annular magnetic ring between one of the coils and the remainder to provide a shunt path for leakage reactance.

4. The circuit ofclaim 6, in which the inverter is a push-pull transistor oscillator.

5. The circuit of claim 6, in which there is a small inductance in series with the alternating current line and the rectifier. 

1. An electric discharge lamp circuit operating from an alternating current line, said circuit comprising: a full-wave rectifier for supplying a substantially unfiltered rectified direct current means to an inverter; an inverter to which said wave is applied, the output frequency of the inverter being substantially higher than that of the alternating current line said inverter including two transistors connected in push-pull; a transformer for coupling at least one fluorescent lamp to the output of said inverter; and input and output circuits of said inverter being coupled through a coil together to produce oscillation at a frequency substantially higher than that of the alternating current line, and an inductance to short circuit said coil when the transistors are not carrying current.
 2. The circuit of claim 6, in which the alternating current line has a frequency of about 60 cycles per second, and the output frequency of the inverter is above 8,000 kilocycles per second.
 3. The circuit of claim 6 in which the transformer is of the pot type and has an annular magnetic ring between one of the coils and the remainder to provide a shunt path for leakage reactance.
 4. The circuit of claim 6, in which the inverter is a push-pull transistor oscillator.
 5. The circuit of claim 6, in which there is a small inductance in series with the alternating current line and the rectifier. 